The Product Box is a visualization of how you are going to deliver the product. If a product is considered as a box what will be in it?

Before developing the product, it could be an exercise by the team to imagine features, Unique selling points. The exercise could be facilitated by the Product Owner/BA. The product Box game helps is assessing how much the team understands the product which is going to develop. The better the team understands the business value of the product, the comfortable they are in developing it and in coming up with valuable ideas for the same.